1. Field of the Invention
Generally, the present disclosure relates to the field of fabricating products, such as semiconductor devices, in a manufacturing environment including process tools exchanging transport carriers with an automated transport system, wherein the products, such as substrates for semiconductor devices, are processed on the basis of groups or lots defined by the contents of the transport carriers.
2. Description of the Related Art
Today's global market forces manufacturers of mass products to offer high quality products at a low price. It is thus important to improve yield and process efficiency to minimize production costs. This holds especially true in the field of semiconductor fabrication, since, here, it is essential to combine cutting-edge technology with volume production techniques. It is, therefore, the goal of semiconductor manufacturers to reduce the consumption of raw materials and consumables while at the same time improve process tool utilization. The latter aspect is especially important since, in modern semiconductor facilities, equipment is required which is extremely cost-intensive and represents the dominant part of the total production costs.
Integrated circuits are typically manufactured in automated or semi-automated facilities, by passing through a large number of process and metrology steps to complete the devices. The number and the type of process steps and metrology steps a semiconductor device has to go through depends on the specifics of the semiconductor device to be fabricated. A usual process flow for an integrated circuit may include a plurality of photolithography steps to image a circuit pattern for a specific device layer into a resist layer, which is subsequently patterned to form a resist mask for further processes in structuring the device layer under consideration by, for example, etch or implant processes and the like. Thus, layer after layer, a plurality of process steps are performed based on a specific lithographic mask set for the various layers of the specified device. For instance, a sophisticated CPU requires several hundred process steps, each of which has to be carried out within specified process margins to fulfill the specifications for the device under consideration. Since many of these processes are very critical, such as many photolithography steps, a plurality of metrology steps have to be performed to efficiently control the process flow and to monitor the performance of the respective process tools. For example, frequently so-called pilot substrates are processed and subjected to measurement procedures prior to actually releasing the associated group of “parent” substrates in order to test the compliance with predefined process margins. Typical metrology processes may include the measurement of layer thickness, the determination of dimensions of critical features, such as the gate length of transistors, the measurement of dopant profiles, and the like. As the majority of the process margins are device specific, many of the metrology processes and the actual manufacturing processes are specifically designed for the device under consideration and require specific parameter settings at the adequate metrology and process tools.
In a semiconductor facility, a plurality of different product types are usually manufactured at the same time, such as memory chips of different design and storage capacity, CPUs of different design and operating speed and the like, wherein the number of different product types may even reach one hundred and more in production lines for manufacturing ASICs (application specific ICs). Since each of the different product types may require a specific process flow, different mask sets for the lithography, specific settings in the various process tools, such as deposition tools, etch tools, implantation tools, chemical mechanical polishing (CMP) tools and the like, may be necessary. Consequently, a plurality of different tool parameter settings and product types may be simultaneously encountered in a manufacturing environment. Thus, a mixture of product types, such as test and development products, pilot products, different versions of products, at different manufacturing stages, may be present in the manufacturing environment at a time, wherein the composition of the mixture may vary over time depending on economic constraints and the like, since the dispatching of non-processed substrates into the manufacturing environment may depend on various factors, such as the ordering of specific products, a variable degree of research and development efforts and the like. Thus, frequently the various product types may have to be processed with a different priority to meet specific requirements imposed by specific economic or other constraints.
Despite these complex conditions, it is an important aspect with respect to productivity to coordinate the process flow within the manufacturing environment in such a way that a high performance, for example in terms of tool utilization, of the process tools is achieved, since the investment costs and the moderately low “life span” of process tools, particularly in a semiconductor facility, significantly determine the price of the final semiconductor devices. In modern semiconductor facilities, a high degree of automation is typically encountered, wherein the transport of substrates is accomplished on the basis of respective transport carriers accommodating a specific maximum number of substrates. The number of substrates contained in a carrier is also referred to as a lot and the number of substrates is therefore frequently called the lot size. In a highly automated process line of a semiconductor facility, the transport of the carriers is mainly performed by an automated transport system that picks up a carrier at a specific location, for example a process or metrology tool, within the environment and delivers the carrier to its destination, for instance another process or metrology tool that may perform the next process or processes required in the respective process flow of the products under consideration. Thus, the products in one carrier typically represent substrates to be processed in the same process tool, wherein the number of substrates in the carrier may not necessarily correspond to the maximum number of possible substrates. That is, the lot size of the various carriers may vary, wherein typically a “standard” lot size may dominate in the manufacturing environment. For example, one or more pilot substrates, which may be considered as representatives of a certain number of parent substrates contained in a certain number of carriers filled with the standard lot size, may be transported in a separate carrier, since they may undergo a specific measurement process and therefore may have to be conveyed to a corresponding metrology tool, thereby requiring an additional transport job. Based on the results of the measurement process, the waiting parent substrates may then be delivered to the respective process tool.
The supply of carriers to and from process tools is usually accomplished on the basis of respective “interfaces,” also referred to as loading stations or load ports, which may receive the carriers from the transport system and hold the carriers to be picked up by the transport system. Due to the increasing complexity of process tools, having implemented therein a plurality of functions, the cycle time for a single substrate may increase. Hence, when substrates are not available at the tool, although being in a productive state, significant idle times or unproductive times may be created, thereby significantly reducing the utilization of the tool. Thus, typically, the number and configuration of the load ports is selected such that one or more carriers may be exchanged at the load port(s) while the functional module of the process tool receives substrates from another load port to achieve a cascaded or continuous operation of the process tool. The time for the exchange of carriers between the automated transport system and the respective process or metrology tool depends on the transport capacity of the transport system and the availability of the carrier to be conveyed at its source location. Ideally, when a corresponding transport request for a specified lot currently processed in a source tool is to be served, the respective substrates should be available at the time the transport system picks up the carrier including the lot and delivers the carrier at the destination tool such that a continuous operation may be maintained. Consequently, the respective carrier should be delivered to the destination tool when or before the last substrate of the carrier currently processed in the destination tool is entered into the process module so that a continuous operation may be achieved on the basis of the newly arrived carrier. Thus, for an ideal continuous operation of a process tool, one carrier would be exchanged while another carrier is currently processed. Depending on the capacity of the tool interface, for instance the number of load ports provided, a certain buffer of carriers and thus substrates may be provided in order to generate a certain tolerance for delays and irregular deliveries, which may, however, significantly contribute to tool costs. In some circumstances, the required carrier exchange time may be negative, thereby requiring a change of the substrate handling scenario.
Moreover, as the actual carrier exchange time does not substantially depend on the lot size, whereas the time window for performing an actual carrier exchange is highly dependent on the respective lot size, since a small currently processed lot provides only a reduced time interval for exchanging (also referred to as a window of opportunity for carrier exchange) another carrier without producing an undesired idle time, the presence of a mixture of lot sizes, such as pilot lots, development lots, and the like, or the presence of lots having a high priority, may negatively affect the overall performance of process tools.
Moreover, in view of cycle time enhancement for the individual products and to address flexibility in coping with customers' specific demands, the lot size may decrease in future process strategies. For example, currently 25 wafers per transport carrier may be a frequently used lot size, wherein, however, many lots may have to be handled with a lesser number of wafers due to the above requirements, thereby imposing a high burden on the process capabilities of the automatic transport system and the scheduling regime in the facility in order to maintain a high overall tool utilization. That is, the variability of the carrier exchange times for exchanging the carriers with respective load stations of the process tools may be high and thus a significant influence of the transport status in the manufacturing environment on the overall productivity may be observed. Thus, when designing or redesigning a manufacturing environment, for instance by installing new or additional equipment, the tool characteristics with respect to transport capabilities, such as the number of load ports for specific tools and the like, and the capabilities and operational behavior of the AMHS (automated material handling system), may represent important factors for the performance of the manufacturing environment as a whole. The handling of different lot sizes within the manufacturing environment that is designed for a moderately large standard lot size may therefore require highly sophisticated scheduling regimes to compensate for the lack of sufficient carrier exchange capacity in the existing tools. However, the presence of small lot sizes may nevertheless result in a significant reduction of tool utilization, in particular in photolithography tools and related process tools, due to the fact that control of material activities in and between process tools are typically performed on the basis of a standardized platform that includes respective rules and models for managing the operation of transport and substrate handling systems in an equipment independent manner. These standard rules and models are specified by SEMI (Semiconductor and Materials International) standards, thereby allowing a machine supplier independent communication and operation of tools in a manufacturing environment.
Based on these standard rules and respective state models, a supervising control system, such as an MES (manufacturing execution system), may exchange messages within the manufacturing environment, i.e., with the respective process and metrology tools included therein, via appropriately designed communication interfaces. Therefore, the supervising control system may be notified about the current status of the respective process tools and may communicate to the manufacturing environment respective messages for managing the overall process flow in the manufacturing environment. The respective messages, after having been translated into an appropriate format by the respective interfaces, may contain the required information for instructing the various components in the manufacturing environment with respect to the operational behavior in order to obtain the desired response of the manufacturing environment. Thus, the information contained in the respective messages may include data with respect to the processing of substrates, and/or may initiate a sequence of tool activities so as to obtain the required process parameters and other actions required for an appropriate handling of the substrate in the respective process tool. For this purpose, various tool activities may be performed on the basis of standardized state models, thereby resulting in a tool independent response to the instructions forwarded by the supervising control system, which in turn may then be notified on the current status based on the underlying standardized state models. For example, in highly automated manufacturing environments, such as a semiconductor facility, most of the substrate handling activities for conveying the substrates within the process tools and between process tools is based on automatic substrate handling systems and transport platforms, which are represented by respective standardized state models as regulated by respective SEMI standards, wherein, for instance, tool internal substrate handling activities for accepting transport carriers, reloading and loading the respective transport carriers in the tool, may be performed in accordance with E84/E87 SEMI standards. According to these standard state models, a transport carrier may arrive at a specific process tool, that is, at one of a plurality of load ports, and may then be unloaded so as to supply the substrates to the tool internal process modules for performing one or more processes required by the specific process flow of the substrates under consideration.
As previously mentioned, in modern semiconductor facilities, not only the quality of the respective processes has to be monitored and maintained within tight process margins, but also the throughput of the process tool is an important factor in view of overall production costs. Thus, it is an important aspect in managing a complex manufacturing environment to supply substrates to the tool internal modules in a substantially continuous manner so as to substantially avoid idle times of the process modules. Consequently, the scheduling of the arrival of substrate carriers is typically performed in such a manner that the substrate carriers may arrive at the various load ports without resulting in undue idle times of the process modules. For example, in typical manufacturing environments for producing semiconductor devices, a lot size is 25 substrates per carrier and the number of load ports per tool is typically selected so as to allow the arrival of a sufficient number of substrate carriers for obtaining a substantially continuous operation of the process modules. According to the respective SEMI standards for controlling the associated substrate handling activities for supplying the substrates from the substrate carriers to the process modules and finally back to the substrate carrier may not allow the removal of the substrate carrier unless all of the substrates have been returned to the respective substrate carrier. That is, presently the substrate carriers used for transport of substrates to respective load ports of process tools have to stay attached to the load ports while the substrates are being processed in the respective process modules. Under these conditions, the continuous process in the respective process tool may be obtained by providing a respective number of load ports, thereby ensuring that a sufficient number of substrates is present in the process tool at any time. However, there is a general tendency for reducing the number of substrates per lot, for instance using 12 substrates per lot instead of 25, in order to significantly reduce the overall process time for a single substrate. In future strategies for operating semiconductor facilities, even smaller lot sizes have been proposed wherein, with respect to flexibility and reduction of overall process time, lot sizes as small as one substrate may be used, in particular if the size of the individual substrates tends to increase. Thus, when reducing the lot size, several process tools may run into throughput problems due to a non-continuous operation of the process tool, since the existing number of load ports may not allow continuous operation. For example, lithography tools and related process modules may suffer from a reduced throughput while many lithography processes represent the most cost-intensive process steps during the entire process flow for substrates. However, simply increasing the number of load ports may be less than desirable due to the significant increase of required clean room area. In addition, a high degree of compatibility with presently installed semiconductor facilities may be desirable so as to allow the presence of moderately large lot sizes, such as 13-25 substrates per carrier, wherein a plurality of lots with a significantly reduced lot size, such as pilot lots, engineering lots, high priority lots with reduced size and the like, should be processed in the manufacturing environment without causing undue throughput reductions in the various process tools.
The present disclosure is directed to various methods and systems that may avoid, or at least reduce, the effects of one or more of the problems identified above.